1. Field of the Invention
The present invention is concerned with electric-powered vehicles having a drive system which includes one or more electric motors controlled and monitored by an electronic control system, and which also includes an hydraulic braking system which is operated by the vehicle driver which operates on at least the front wheels of the vehicle.
2. Discussion of the Background
Drive systems in electrically powered vehicles are usually of one of two types, namely the type where there is a respective electric motor drive unit associated with each of a plurality of wheels (referred to as motor-in-wheel drive units) and the type which has a single central electric motor and gearbox, with a conventional differential and drive shafts. The present invention is applicable equally well to both of these motor drive configurations, although the motor-in-wheel arrangement is probably preferred.
For deceleration of such a vehicle, in addition to the hydraulic brake torque M.sub.Hydraul, which is effected by the driver-operated brake system, an electric brake torque M.sub.Electr. from the electric propulsion motor(s) is available and usable. This arises from the well known effect that certain motors, when mechanically driven with the energizing current removed, operate as dynamos and generate a current but with a resultant braking moment acting against the applied mechanical effort, thus effectively acting as a brake.
As a consequence, the total brake torque M.sub.Brake on a vehicle wheel is given EQU M.sub.Brake =M.sub.Hydraul. +M.sub.Electr.
Since a vehicle battery has only a limited available capacity, it is found useful to charge the vehicle battery by supplying to it the "dynamo" current generated by the electric brake torque M.sub.Electr. This process is referred to as "regenerative braking".
With such a braking system, a simple anti-lock (ABS) braking arrangement can be realized modulation of the electric current applied to the electric driving motor(s) and hence of the brake torque.
With a conventional hydraulic ABS, the brake torque is proportional to brake pressure. Similarly, electric brake torque is largely proportional to electric motor current which can be controlled by, for example, pulse width modulation techniques. Thus, if excessive wheel slip occurs, the electric brake torque is arranged to be reduced. After wheel recovery, the electronic control unit (ECU) controlling the braking system reactivates the electric drive. The ABS algorithm can be substantially the same as for hydraulic brakes and vehicle performance will be maintained if electric motor torque reduction is sufficient for wheel recovery (see accompanying FIG. 1).
Anti-lock (ABS) braking is always required if the brake torque .sup.M brake exceeds the anti-lock torque M.sub.lock at which the wheel locks and which depends inter alia on the coefficient of friction between the tire and the road surface, i.e. when EQU M.sub.Brake &gt;M.sub.Lock
For anti-lock braking, two basic cases must be distinguished. First, in the case that EQU M.sub.Lock &gt;M.sub.Hydraul
which occurs on road surfaces of high coefficient of friction (high mu), the modulation of the brake torque M.sub.Brake required for the anti-lock operation can be effected by variation of the electric torque component M.sub.Electr by controlling the electric current supplied to the relevant electronic motor. In this case, the anti-lock procedure would be executed wholly within the "regenerative range" of operation, without additional consumption of electrical energy. If, on the other hand, M.sub.Lock &lt;M.sub.Hydraul which may occur on surfaces of low coefficients of friction (low mu), the electric motor has to be arranged to produce a counteracting torque M.sub.Contra (=M.sub.Lock -M.sub.Hydraul) because the wheel must be driven against the hydraulic torque M.sub.Hydraul if wheel lock is to be avoided. This procedure would happen outside of the "regenerative range" and cause an additional consumption of electrical energy.
Such a process would in principle still be practicable in the case of an extremely small electrical counter torque M.sub.contra, but in the case of an extremely low coefficient of friction (adhesion), which would occur for example with a road surface covered with snow or ice, and hence producing a very small anti-lock torque M.sub.Lock, such a procedure would be inoperative because the counter torque M.sub.Contra to be overcome would be too great.
As the advantages of anti-lock systems are mainly effective in the case of low coefficients of adhesion, it would thus be necessary to include already known means for the purpose of hydraulic pressure modulation, which would result in disadvantages such as an increase in weight, a requirement for additional installation space, an increased expenditure in connection with the assembly and hence an overall increase in costs.